2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team
(Read Full Article:
"Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko" )

Sunday, July 08, 2012

Quantum Gravity: Can It Be Empirically Tested?

Claus Kiefer (left) and Manuel Krämer (right)

[Every year (since 1949) the Gravity Research Foundation honors best submitted essays in the field of Gravity. This year's prize goes to Claus Kiefer and Manuel Krämer for their essay "Can Effects of Quantum Gravity Be Observed in the Cosmic Microwave Background?". The five award-winning essays will be published in a special issue of the International Journal of Modern Physics D (IJMPD). Today we present here an article by Claus Kiefer and Manuel Krämer on their current work.
-- 2Physics.com ]

Quantum theory seems to be a universal framework for
physical interactions. The Standard Model of particle physics,
for example, is described by a quantum field theory of the
strong and electroweak interactions. The only exception so far is
gravity, which is successfully described by a classical theory:
Einstein's theory of general relativity. The general expectation, however,
is that general relativity is incomplete and must merge with
quantum theory to a fundamental theory of quantum gravity [1,2].
One reason is the singularity theorems in Einstein's theory, the other
is the universal coupling of gravity to all forms of energy
and thus to the energy of all quantum fields.

Despite many attempts in the last 80 years, a final quantum theory
of gravity is elusive. There are various approaches, which
all have their merits and shortcomings [1,2]. A major problem in the
search for a final theory is the lack of empirical tests so far.
This problem is usually attributed to the fact that the Planck scale,
on which quantum gravity effects are supposed to become strong,
is far remote from any other relevant scale. Expressed in energy units,
the Planck scale is 15 orders of magnitude higher than even the energy reachable
at the Large Hadron Collider (LHC) in Geneva. It is thus hopeless to
probe the Planck scale directly by scattering experiments.

In our prize-winning essay [3], we have addressed the question
whether effects of quantum gravity can be observed in a cosmological
context. More precisely, we have investigated the presence of
possible effects in the anisotropy spectrum of the cosmic microwave
background (CMB) radiation.

But given the presence of many approaches, which framework should one
use for the calculations? We have decided to be as conservative as
possible and to base our investigation on quantum geometrodynamics,
the direct quantization of Einstein's theory.
The central equation in this approach is the Wheeler-DeWitt equation,
named after the pioneering work of Bryce DeWitt and John Wheeler
[4]. It is a conservative approach because the Wheeler-DeWitt equation
is the quantum equation that directly leads to general relativity
in the semiclassical limit. It possesses for gravity the same value
that the Schrödinger equation has for mechanics.

While the Wheeler-DeWitt equation is difficult to solve in full
generality, it can be treated in an approximation scheme that is
similar to a scheme known from molecular physics - the
Born-Oppenheimer approximation. It basically consists of an expansion
with respect to the Planck energy. It is thus assumed that the
relevant expansion parameter is (the square of) the relevant energy scale
over the Planck energy. A Born-Oppenheimer scheme of this type
has been applied to gravity in [5]. In this way, one first arrives
at the limit of quantum field theory on a fixed background.
The next order then gives quantum-gravitational corrections that are
inversely proportional to the Planck mass squared. It is these
correction terms that we have evaluated for the CMB.
The quantitative discussion, on which our essay is based,
is presented in [6]. We assume that the Universe underwent
a period of inflationary expansion at an early stage and that
it was this inflation that produced the CMB anisotropies
out of which all structure in the Universe evolved.

What are the results? The calculations show that the
quantum-gravitational correction terms lead to a modification of
the anisotropy power spectrum that is most pronounced for
large scales, that is, large angular separations at the sky.
More precisely, one finds a suppression of power at large scales.
Such a suppression can, in principle, be observed.
Since up to now no such signal has been identified,
not even in the measurements of the WMAP satellite, we can find
from our investigation only an upper limit on the expansion rate
of the inflationary Universe. The effect is therefore too small to be
seen, it seems, although it is expected to be considerably larger
than quantum-gravitational effects in the laboratory.

A similar investigation was done for loop quantum cosmology [7].
It was found there that quantum gravitational effects lead to
an enhancement of the power at large scales, instead of a suppression.
These considerations may thus be able to discriminate between
different approaches to quantum gravity.

What are the implications for future research?
It remains to be seen whether the size of quantum-gravitational
corrections terms can become large enough
to be observable in other circumstances.
One may think of the polarization of the CMB anisotropies
or at the correlations functions of galaxies. Such investigations
are important because there will be no fundamental progress in
quantum gravity research without observational guidance.
We hope that our essay will stimulate research in this direction.

2Physics.com publishes invited semipopular level articles on key developments
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